EXP is shared mid so he'll level a lot faster than them, will do more damage than them, and can probably kill them early on. The longer the game goes, however, the more they'll catch up and the smaller his chance of winning.
No it was a for fun match with my friends (they're probably decent ELO from what I can see). It was just that they decided they'd try and towerdive at the start and I had fortify. Early rod > more early kills > GA+Rylai > more kills > can't push after killing everyone > lose game due to no push