Lately, I have been wondering about the divorce rate in the church these days. I have read the all the verses posted. My question would be, what if a believer marries the adulterer, is that believer committing adultery?
I am taking this question as a general one and not referring to your second question.

Thus, if a believer marries an "adulterer", i.e., someone who has committed adultery while previously married and is divorced on the charge of anything but adultery or desertion (unbiblical divorce), then the answer would be, "Yes". Having said that, it must also be understood that should the believer repent, i.e., with a genuine heart-felt acknowledgment that he/she knowingly married the adulterous spouse, forgiveness is given by God and should be by men.
The second question I would have is this, (an actual incident) ....a person (man) who was a non believer marries someone else's wife, then becomes a believer during this marriage, gets convicted, and gets divorced from this wife, and she goes back to her original husband, and now this believing man calls himself 'single'...and says that God never acknowledged his marriage because she belonged to her first husband. What do you think about his reasoning? I'm really confused on this matter....thank you...
Sin is often complex and the results can be a 'sticky wicket' to work out. Let's see if I can express how I would view and deal with this actual situation.
1. The man committed adultery by marrying a woman who was still in covenant bond to another man. I am further
assuming that he was aware that she was still married.
2. The marriage, at least in western society, would have been considered polygamous and illegitimate. I am not 100% sure what Scripture would consider this polygamous marriage, i.e., an actual marriage or simply adultery.

3. The return of the woman to her original husband, I believe, was the right thing to do. This would imply, of course, that I would therefore consider her second marriage to be illegitimate and thus an adulterous affair.
4. Considering the above, I believe the man would be correct in considering himself to be "single", i.e., the adulterous relationship was not a legitimate marriage, at least according to civil law (polygamy). However, he would nevertheless be guilty of adultery, which again should not be deemed an "unpardonable sin". The problem, however, wouldn't end there because there is the matter of his marrying another. Are you beginning to see how complex and difficult this type of situation is?

5. One last item which presents itself is the woman's returning to her original and legitimate husband. IF the second marriage is considered to be a true legitimate marriage, then the former husband to whom she returned has sinned in taking her back (Deut 23:3,4). But IF the second marriage was not a legitimate marriage, then if the former husband has it within his heart to forgive her, should she petition him to do so, repenting of the evil she had done, then his taking her back would be sanctioned. Whew!

These are my initial thoughts only and I would be open to change after more consideration should I discovered that Scripture had more to say in regard to this particular situation.
What have been your thoughts? I know there are others who would disagree with my assessment with perhaps good arguments or spurious arguments.
PS. Depending upon other details, the assessment of the situation could change.

PSS. It seems that there ARE essential details which were not given in the original post by "EternallyHis" of which I have come to learn. It is unfortunate that these details were not made known at the beginning because my response would have been much different.
Taking into account this new information, which even so was not complete, and once again
assuming they are factual, I can offer the following:
1. Since the woman's first marriage was dissolved by divorce [no information was given as to whether it was a biblical divorce or not; adultery and/or desertion], the subsequent marriage was legitimate, i.e., it was a true and binding marriage, even though it might have been an adulterous marriage.
2. It makes no difference if the man of the woman's second marriage was a believer or unbeliever when the marriage occurred. IF the marriage was, in fact (not known at this time), an adulterous marriage due to an unbiblical divorce of the first marriage, this can be repented of by the offending parties. The covenant bond of marriage is not effected; it is still a valid marriage between the two parties.
3. The reason for the divorce of this second marriage is also not revealed so it would be impossible to determined whether it was legitimate (biblical) or not.
4. REGARDLESS... what is true is that the man in question is NOT "single" because his wife left him to return to her former husband after 28 years of being married to him. Put another way, it is NOT true that his marriage to this woman can be said to have never happened, i.e., there was no real marriage, and that makes the man "single". He is single due to the divorce, but whether he can be said to be the innocent party is not possible due to the lack of information on the grounds of both the first and second marriage.