wedding planning

I know traditionally the bride and her family (if she's fortunate to have a family who can) are usually the ones who pay for the majority of the wedding. I've been doing research and see that traditionally the grooms family would pay for the rehearsal dinner which we are not having. Luckily my parents have paid for most of our wedding thus far and my fiancés mom who is not as well off has not paid for anything..I know she doesn't have much to contribute but I'm feeling very guilty that my parents have done soo much for us and she hasn't. Since we aren't having a rehearsal dinner, do you think it'd be unreasonable or rude for my fiancé to ask her to help with paying for his tux?