Go read the ANO.
You can use someone else's aircraft providing no "valuable consideration" is given or promised for the flight or for the purpose of the flight.
If you own at leat 5% of the aircraft you can pay towards the direct operating costs.
If he lends the a/c to you and you put fuel in you'd be OK. If he lends you the a/c and you pay a credit into his fuel account for more fuel than you used you'd not be OK.
If you do a deal with him that says "I'll pay for your ski holiday if you let me have 7 Hrs in your a/c" then that 's probably illegal because valuable consideration is being promised.
This advice is worth exactly what you have paid for it.
Mike