Ask the seller to provide some sort of paper trail ie receipts for servicing, repairs, spare parts etc - as much paperwork as possible via "the friend"
When viewing the boat scrutinise any paperwork you come across ie names, addresses, phone number, email address etc. See if these tally with the seller.
Make sure "the friend" has all original keys for every lock on the boat.
On the day of the viewing, post a pic of the boat on here, maybe a member here knows the boat and or owner (small chance)
Re starter battery, take a set of jump leads along in case the battery doesn't fit or cables don't reach.
Good luck