I like Softails, think the Softail Custom is one of the cleanest looking models H-D ever made, really has that 60s-70s look, & you can't beat an Evo engine. So if its in good shape, I'd offer $4k and use tires, oil change and check up as the excuses of why I'm offering less. But because I like that model, I'd probably pay her price if push came to shove, lol. Hard to find a good big twin of any year under $6-$7k.
I make it a point to never buy/sell from or to friends. If it turns out to be a basket case there is a good chance of troubled waters between you both.
A little more info would help with the assist. Any upgrades/mods? When were the tires and battery replaced. Is there a maintenance record? Was it ever damaged? And a big thing for us in the northeast, was it ever in water (think Irene, Lee, and Sandy)
Just be aware that parts for an '86 are to find sometimes. Also few shops know how to work on them. But they're near classic bikes and if in original shape are good buys.