Paul,
I'd just get a new battery for your friend's board and use it. If it was originally a SuperDrive/FDHD board, it will likely have the battery holder, so you can use a generic Mac PRAM battery in it. On the 800k boards, there is someone on eBay selling a triple AA battery pack with leads for the Apple IIgs computers, which will work fine when/if the old Varta batteries do finally kick the bucket on an early SE or II/early IIx.
-J
I'd just get a new battery for your friend's board and use it. If it was originally a SuperDrive/FDHD board, it will likely have the battery holder, so you can use a generic Mac PRAM battery in it. On the 800k boards, there is someone on eBay selling a triple AA battery pack with leads for the Apple IIgs computers, which will work fine when/if the old Varta batteries do finally kick the bucket on an early SE or II/early IIx.
-J