Yup the uncle would have CGT, depending you what his circumstances are. The uncle may be able to avail of same event relief, i.e. giving credit for the CAT paif by your partner.
You could in theory avoid the tax liability on the site, however in order to do so you'd have to do a lot of tax planning....
The idea would be to gift the site as a going concern and for your partner to claiming the favoured nephew/ niece allowance. By doing this your partner's uncle may also be able to avail of retirement relief on the sale.
It's just a suggestion but I'd imagine as the tax liability is so low it'd be more of an inconvenience than it's worth.
Is your partner working?