ok so griffin thought that turning visible and invisible at will is impossible but if griffin can make himself invisible then it's possible to do it at will
griffin in those 3 panels was thinking this
1st panel: "i'm not so sure about this but being able to turn invisible at will sounds helpful" (by helpful i meant as in daily life stuff or messing with annoying ponies not hurting them or getting away from ponies in general griffin is smart to know what happened to him)
2 panel: "but i know what happened to me, i don't want to die again so i don't what to hurt anypony i might as well accept the offer"
panel 3: he accepts he offer
Matthew is relived but now wonders what happened to griffin, knowing what happened to him and how griffin felt Matthew does feel pity for him (it's the same way griffin told kemp his story only Matthew is a bit more understanding yes Matthew knows what griffin did was wrong but understands that griffin went crazy and feels more sorry for him really though he Matthew may be a bit nervous but he will try to how do you say, keep his cool or to cool down really)
but soon griffin ask where he can find cloths and you will see it in the next page soon