If the filed a suit against you, you can counterclaim the invalidity of the asserted patents. And even if they drop the claims against you, you still have the claim that their patent is invalidity.

If they just threaten you, can still sue the patent owner for a "declaratory judgement" that their patent is invalid, though you have to show harm in order for the court to hear the case. So if they promise not to sue or your customers, the case may get dismissed.

You can always file an Inter Partes Review at the patent office to attack any patent.