I am not sure if I have missed something here, but surely each of these two people is benefitting from the other one's financial contribution.
It is therefore in both their interests to have the other one coughing up.
If this arrangement doesn't work, then the partnership (and there is a partnership, written or not) can be dissolved. The asset can be simply sold and that's it.
If Gordon's mate refuses to buy Gordon's share out then he is taking the micky, and I would challenge him directly with the threat of dissolving the partnership.
Regarding lawyers, the one thing I have observed here (and on other forums) for the past few years is that no lawyer ever offers specific advice on anything for which he could charge for... Most notably, nothing is said on actual cases of alleged ANO breaches which the said lawyer must have dealt with countless times, yet the pilot forums are packed with "is this legal" speculation.