If you can convincingly show prior use of the patented stuff, with
independent development *or* with *them* copying *you*, you can get
their patent voided.
If you can show use prior to the date they claim they started
developing it, that is sufficient proof of independent development.
If you can show that they copied you, you might be able to take the
patent away from them.
Be warned that *any* of these will take a significant court battle
and, except in the latter case, you probably can't get a court to
make them pay your legal costs.
Suggest you consult a genuine patent attorney on this.
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