They stand up and are engaged in combat.
Under the 2023 rules fighters subject to the blaze condition don't get reaction attacks (page 68, 2023 book), so the -1 to hit back they'd normally have from getting up from being pinned to fight is irrelevant.
If they survive the attacker's post-charge fight action they get to scream then hope their comrades eliminate their opponent, otherwise on their next activation they take the usual S3 Ap-1, D1 hit from being on fire, then must attempt to put the fire out while standing (as they cannot go prone while engaged- that's a move-equivalent action they don't have access to). Hopefully by then they're surrounded by buddies who will give a +1 to the douse fire process (requiring a roll of 6 on a D6).
TL;DR version, they're a very easy target that can't fight back and has a minimum 17% chance of not being so when they next activate (if the burning doesn't make them Seriously injured first).
