Is there a money trail at the time, of funds flowing from them to you for their share of the acquisition?
The cynic in me looking at it in the absence of any other facts, would see an opportunity for 2 people to engineer a situation where the one with a capital loss and no gain to use it against, and the one with a capital gain that they'd like to reduce, will both benefit (with a few quid of the tax saved passing under the table). The kind of tax avoidance scheme one might dream up in the pub of a weekend...!