This is coming from a guy that has played Dota a grade total of one time so don't take my word as final say. I think a gtx 750ti or an R7 260x should be able to play it easy.
I was playing Dota 2 at 720p on some ancient school computer with onboard graphics, it went alright.
The main issue with your friends rig is making sure that the power supply is decent enough to sustain a discrete graphics card. If the power supply has a six pin and you want a discreet card I'd go for the 750 ti.
Personally if I had the money I'd be upgrading the whole thing.
750ti should be good. You'll have a limit with that set up u currently have. PCI-E 3 is out of the question and most likely you'll have problems even in Gen 2 so don't expect a massive upgrade from the 750ti. The CPU is missing a few strings required for regulation.
The one thing that's gonna suck is that he'll have to either get a new computer later on because that thing is OLD and new gen games require certain instructions like SSE, AVX, etc. That computer is running on fumes i can assure you of that.
I allready have a new computer , this one i would only use for dota when friends would come over so i would not need it to do anything else . If an old gpu would do the trick i would try and buy a used one