It would depend on who they are. If they are someone whose opinions I absolutely despise on GAG, I'd tell my friend about those opinions, as a heads up.
If they were someone I asbolutely adore on GAG, I'd tell my friend anyways, just to let them know what awesome new person they have met.
Your question made me wonder what it would be like, if I met certain GAGers in real life. Let's see...
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If I met DodgersGM in real life, I'd hang out with him at some snugly place, have a beer (or two or three or four...) and listen to Modest Mouse and Interpol and Weezer for hours and hours nonstop.
If I met bubbiexo in real life, I'd just hug her and not let go of her until the fire department seperates us with whichever hyraulic rescue tool gets the job done, then we'd just talk and talk and talk and have an amazing time.
If I met Kris85 in real, we'd have a couple of drinks, then hit some club, have a couple more drinks and go nuts.
If I met ImCoasting in real life, I'd comb his mustache, stroke his skull and gently say to him, "ImCoasting".
If I met RedThread in real life, I'd grab a ladder and climb up to where his face is, puff away the fair weather clouds and say, "Nice to meet you, fine Sir. How are you doing on this magnificent day?", and see where it goes from there.
If I met sparrow24 in real life, I'd just put my hands in my pockets, look down to the ground and mumble, "Sorry, ma'am."
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These are just a few examples of course. I will not tell you though what it would be like, if I met one of the GAGers I absolutely despise. I don't want my answer to be removed. ;)