What to do if my friend was ordered to pay his ex-wife a settlement check but his attorney will

not accept the check unless it is made out to his ex-wife and her attorney?

Some attorneys have clauses in their contracts which provide that an attorney's outstanding bill get's paid off before any funds are distributed to the client.  This is why the attorney wants the payment in his name as well as the client's name... they want to get paid.
However, regardless of the attorneys preference, the order which imposed the settlement obligation will control.  If the order says he is to pay the funds directly to wife, then your friend needs to send the funds directly to his wife.  If the order says that he is to tender a check to his ex-wife by delivering it to her attorney... then he needs to write a check payable to wife and mail it certified to the the attorney (but not payable to the attorney)   If the order says that he is to tender a check payable to the attorney and client... then he should tender the payment to the attorney payable to the attorney and the client.
Bottom line.... the order sets out how, who, and when and that's exactly what your friend should do.  If your friend is not sure how or who is supposed to pay because the order is confusing, then your friend should consult with an attorney.  Many counties now offer free legal clinics where people can show up and receive assistance with basic questions.
If the order just says that your friend is supposed to tender funds to his ex-wife, then he needs to send the funds to her via certified mail.  That will give him proof that he mailed the check.  The form of payment needs to be a check, so that when it's cashed, he has proof of receipt of funds.  The check should be accompanied with a letter that basically reads:  Enclosed, please find a check no. ______ in the amount of $_______ as satisfaction of the settlement ordered to be paid by the order entitled '__________', page ____ as satisfication of said settlement.  He should make a copy of the check and letter before he mails it and then keep an extra copy in a safe place should any issues arise later on.  He can use the check and the letter to prove that he satisfied the settlement obligation.  If they chose to reject the check, that's on them... he main thing is your friend has fulfilled the act of tendering and submitting the payment.