A jumpsuit isn’t necessarily informal. We’re not talking about a body con jumpsuit. This is a structured jumpsuit. Dressed up with heels or nice flats, statement jewelry, etc., this is definitely an appropriate wedding looks for a 23 year old.
Why are you and others adding additional information that the OP never said? If this was a traditional wedding that required a certain dress code she may not be familiar with, or black tie event that required a more formal look then he would have used that as justification for why he thought his girlfriend should change. Literally all he said was that she’d look stupid as she would be the only one wearing pants.
There’s a minimum expectation of a wedding attire—unless the invite was specific as to dress code, then the jumpsuit pictured meets expectations—it just doesn’t meet his personal expectations and that’s an entirely different issue.