Can I tell my friend to not adopt a dog?
So you are serious? Why don't you want your friend to adopt a dog?
The dog will miss him all the time when he's away. I think my friend will suffer from dealing with the dog too.
I don't think that you should get involved in the situation. It's rude to tell people what they should and shouldn't do. They could have thought through all the scenarios first, so you shouldn't give your opinion unless asked.
I'm sure they will regret it later.
This is not your situation to judge. Maybe you're being too negative. Try and see things from their standpoint, would you want someone telling you not to adopt a dog if you wanted one?