Motorcycle can be shielded, and I think as Wario turns his motorcycle around, the active hitbox that would otherwise hit you, is gone for a moment. So if you shield > up smash, that might work against him. I did this with my Samus when I was fighting a Wario once. Don't expect that to always work, though, because I don't know if the motorcycle's hitbox is still lingering when it turns around. I was just near the edge of the stage when I decided to try that option, and it killed him. You can also do a full/short hop > down/neutral air against Wario as he drives his bike, assuming he doesn't jump off of it first before you use that option. Some Wario's I've faced would use bike > Chomp, because they knew I'd shield their motorcycle, and Chomp ignores shields, will still get you if you dodge (not airdodge), or try to counter it with a neutral attack. There are cases where a neutral attack versus Chomp will result in sort of a grab release animation for both characters.
Chomp can be easily dealt with if your friend is being predictable with it. So if he's the type to use the bike and then follow up with a Chomp, then airdoge or space the Chomp, and punish him by attacking his backside, or from above him. Just be sure you don't attack above your opponent if he blocks a lot, because then he'll counter it with a grab or attack.
