I've had this happen to me several times. Guys in my class (who I barely knew) would introduce me to their Moms and I would never get suspicious. I just figured Well you can't turn down someones Mum can you? Lol
But Deon was asked "What do you do when girls ask where your Mom is?"
He said that by that time, they'll be eating dinner. He has a point lol. He could say something like "Oh lets grab a bite to eat before you meet her."
Then later, you'd be like "Are we going to meet her now?" And he could basically make up something like "Oh I didn't tell you she called an hour ago and said maybe another time."
By then, the girl will probably say "Well anyways, this was fun. We should do it again sometime!"