Worse case scenario, you buy a 80 gig drive, install ME on it but its only recognised as a 32gig drive because of the old BIOS. Solution then would be to flash the BIOS but if you try that and it goes wrong then the motherboard will be useless. Other option just use it as a 32 gig drive and don't flash the BIOS, wouldn't do any harm. If you have a look here as an example at the top 2 click here the difference is price is only £2 not a lot really. I'd go for the 80gig one. If its recognised as 80 fine, if not use it as a 32 but I wouldn't flash the BIOS. If your friend buys a new PC later on he can always use the drive with that as an 80.
No problem with milenium, it will take any size drive unlike win 98. I asked the same question to a BT it engineer, his answer was 'It will take whatever size drive you want to put in'. I was thinking of getting a 120 gig drive at the time and he said it would be no proble as Win ME can handle it.
and this shows how the bios affects the size of hard drive you put in. click here
I have had a scan around the net. The original motherboard would only support a 8.4 gig hard drive. I cannot find any information about the 4.1 release date. In fact nearly all reviews are post December 1999.
From that, I can just guess. I would say that 32gig may be the largest you can use.
There is another solution. Some hard drives come with size limitation jumpers. They are usually 32gig. So you could buy a 60gig and try. If it did not accept it, you could use the limitation jumper so it would boot with just 32gig.
Finding a 32gig hard drive is neigh impossible, the nearest would be 30gig and they can be difficult to locate. The actual cost of a 60gig to a smaller drive is minuscule. This may be option you want to consider.