It is a conflict of interest to do the tax returns of two different individuals (most commonly divorced or separated partners) who have differing or opposing views on the tax treatment of certain items. The accountant may prepare these returns if he or she has written consent from both of them and has disclosed the conflict. There are times when two divorced persons may use the same accountant and he or she is not aware of this, or that two divorced persons do not have competing interests, and in this case no conflict would exist.
I personally try to avoid doing the returns of both divorced or separated spouses.
I am not exactly sure what is going on here, but it appears that your husband is currently paying alimony. It appears that the accountant is suggesting that if the ex-wife has less income (does not report the alimony as income) she will pay less tax, and your husand doesn't deduct the alimony--he will pay a little more.
This is highly unethical as well as illegal---never mind the conflict of interest. Being a party to this is fraud (your husband not deducting the alimony with the intent to assist the ex in paying less tax), and knowingly not including taxable income on a return is considered tax evasion. In addition, the accountant can be sanctioned or fined for encouraging this. If he is a Circ. 230 practitioner (EA or CPA) he could face more significant repercussions.
In addition, it does appear that there is a conflict of interest between the two ex's----the accountant is "milking" them and their situation for his own financial gain.
I would advise your husband to have nothing to do with this scheme and to find a new accountant immediately.