In all candor, your friend would be better off checking and cleaning the starter and ground connections after verifying the generator performance. Unless the charging system is set for it, the 8.5 volt battery will not be kept a full power by a stock 6V system.
I agree with Fortyniner clean,replace. I have a Ford 8N and the first thing I did was replace the ground with a big fat braided ground cable and a heavy duty 6-volt covered cable to the starter. I used a wire brush on a drill to clean the spots where connections are made, tractor now starts in half a turn.
8 or 8.5 batteries are Band-Aids and, as stated earlier, will not get fully charged by a 6 volt generator. As also stated earlier, clean and solid grounds with the correct size cables will make the 6 volt system work fine, unless there is another problem like low compression, etc., but that's a different situation altogether.
8-volt batteries were common to some older 32-volt golf cars and old farm wind generated electrical systems. 4 batteries will give 32 volts and 15 will give you 120-volts. Very little runs on 32-volts now days and very little runs on 8-volts.
These batteries are not common to automobiles. Even back in the 60s it was hard to get good quality large gauge copper cable for the olf 6-volt cars & trucks. If the cables were replaced with too small a gauge or are corroded, it will not function well at all.
WOW! You have a nice car, very lucky to have that. You did a fine job on the restoration. I figured if you had it done it was going to be a treat to look at. Thanks for the photos.