Just an idea, but you could email the mother saying how happy you are to have helped her daughter find her bashert, and how appreciative you are that she sent the invitation/check. However, you have been watching your mail carefully, and you are concerned because they never arrived. Ask her if she could contact her bank to see if the check was cashed, because if it was, it was stolen. If not, it was lost in the mail, and either way, you would greatly appreciate it if she could send you a replacement check for your efforts.
Good luck…