Hi, does anyone know if it can be done? It is a waterproof 3 in 1 Parka by Jack Wolfskin. Pretty similar to the North Face waterproof shell jackets.
An alteration place said they could do it but I am not sure if I trust them or not.
If you shorten the sleeves of your jacket, you risk puncturing the fabric which would prevent it from being waterproof. You'd end up with very wet wrists! I know most outdoor brands tend to put velcro straps on the cuffs of their waterproof jackets to allow you to shorten the length of the sleeves - do your cuffs not have these?
If you have the sleeves shortened it will also void your warranty, which might cause you problems in the future.
If you really do want the sleeves shortening, send an email to Scottish Mountain Gear. They repair faulty outdoor goods, but do it without damaging the fabric. They might be able to help. While I'm sure the alteration service you spoke to will have had the best interests, I'm not sure your typical high-street alteration shop would be able to make the changes without permanently damaging the fabric!