If you use a TV Tuner card to do the conversion, make sure you have a good speed system and reasonably fast hard drive, with plenty of space. You have to make tradeoffs. You can search the web to get more details, but basically it's this:
To record at best framerate (30fps) without compression, you need LOTS of hard drive space as well as it has to be a very fast hard drive. 35gigs an hour I believe in straight AVI format. There's simply a LOT of data that has to be written to the drive; if the drive can't keep up speedwise, you will lose frames. However you get perfect quality in the frames that don't get dropped, and can later compress them as much or as little as you want.
If however you'd like to use compression such as MPEG2 or Divx, then you need a very fast processor to keep up, but you need less drive space and the drive doesn't need to be quite as fast (still needs to be fast though). If the processor can't compress fast enough, it'll drop frames. This is usually the best option, however you're limited by the speed of the processor and drive. If you try to use extreme compression, the processor won't keep up, but less compression means more drive speed and space needed. Usually people use a compromise.
My Athlon MP 1.2GHz and 7200RPM Maxtor ATA100 DiamondMax60 has problems avoiding dropped frames at 30fps, so there's something of a comparison for you.
There is another option. There are external hardware converters, but they usually require something like a Firewire or DV connector. The Dazzle line has several. The good thing about these is they do all the work in hardware, and send the final video to your computer. The downside is they're often limited in terms of bandwidth. They're connecting via USB or Firewire connectors. USB of course has extremely low bandwidth. Firewire is better, but more expensive. Usually you're limited to a certain size video, and 15 frames per second, for example. (Most TV tuners won't let you capture above 320x240 anyway, depending on the model and software used.)
Of course, if he had unlimited funds, your friend could get a professional system that is designed to do nothing but convert VHS to digital. However I wouldn't even know where to look for one of those except maybe a TV studio.