Cost of replacing phev battery outside of warranty?

My partner bought an x3 phev hybrid from an auction site, he keeps saying it was a great deal.

The car drives hpi clear BUT is just over 100k miles so outside of warranty.

We have discovered issues with the hybrid battery it says it’s fully charged but essentially doesn’t work.

He is still saying this was a good deal I don’t believe him him more.

How screwed are we, how much to battery replacement or repairs cost

We purchased it for £14k