I'm thinking around $3k give or take a few hundred depending on condition.

Xenon unshopped but working ~$1800
Black Knight unshopped non-working ~$1200

I'm also factoring in the purchase of both games at once, they might be able to get a little more selling them separately.

Tough question without more details.

Condition of the boards - better pictures of the backglass and playfields.

Pinside has the value at:
Xenon - $1800
BK - $2100

Of course that assumes they are working and in good shape which one is not working.

The other thing to consider is how quickly do they want them gone. They could ask $5k for the pair and wait 6 months for someone to make an offer or sell them quickly for $3k like Frunch suggested.

Have him throw in that dollhouse and make it $2500 for the pair.

I assume you've thought about how hard it may be (or not) to get them out of the basement? Figured there was a reason why the sellers left them with the house.