Laws vary from state to state but there are rules. Ask at your local city or town hall what the rules are to perform a marriage and see if this person complies with those rules... i.e. is he/she licensed to perform a marriage?
Becoming "legal" to perform wedding ceremonies is ridiculously easy. I got my ordination online in less than an hour and had one couple that had to turn all the paperwork into the US Immigration office and it was legal enough for them. Check with your county clerks office and ask your preacher for his certificate of ordination...make sure it all complies with your state laws. The county clerk can help you with that.