UK Tier 2 visa - passport validity requirements on a reentry [closed]

If a person (friend of mine) holds a Tier 2 UK visa (actually, a spouse variant of Tier 2) and their passport is expiring within a couple of month - can that person still exit and enter the UK freely before obtaining their replacement passport? The person can't enter the UK visa-free, if that matters. The gov.uk website doesn't give any info on that, or I'm really bad at searching it properly. I assume the expiry date doesn't matter if you've got a valid visa, but I'm not 100 per cent sure...