Badly in the woods, better up the hills, not round corners.
Its probably best to assume that licence free (PMR 446) use is line of sight, up to about a mile or so, and anything better than that is a bonus. So for OP’s drone application (what are you actually doing?) they might well be fine (line of sight straight up?).
Trees are likely to affect RF propagation (how much exactly is hard to say), but the effect is greater at higher frequencies (e.g. GPS signal gets lost in the woods quite easily). Your other consideration is the power of license free transmission (0.5 W isn’t a lot to lose).
You could perhaps consider a directional antenna for better reception if you knew where your friend was likely to be – this is better suited to ferals than Trimix I think.
edit – they can type faster than me!