Weight is technically less important. She climbs and flies slower and accelerates longer if heavy. You can factor this into your calculations.
No technical problem here, rather a legal one. If nothing happens, nobody cares. If you will have a small accident and metal will bent, you will get probably problems with insurance. Not to speak if you will hurt somebody.
Much more important is the balance. You have to keep it well within limits. Fuel consumption will be higher.
The plane feels the same if slightly over gross. The performance numbers are worse however, most important is the ground roll and climb performance. Best glide speeds, approach speeds and the like will be higher.
And do not try any unusual maneuvers, fly very defensively, you are not a test pilot. If you will scare your girl and her parents it will have negative consequences in the relationship! Being you I would want to make some hourbuilding first.
I would rather take the risk of being heavy than marginal with fuel...
I have a C172SP. I take my wife and daughter and her boyfriend for longer trips. I can normally fly four of us and very light luggage with almost full tanks too. It is not legal but technically OK. If I remember correctly I was ca 70 kg over limit at take off.
My remarks are concentrating on technical aspects. Final decision and
responsibility is yours you are the PIC. Lives of your passengers will be in your hands.
Miroc