Well you need to find out what the fees would be for the debit card.

Using a credit card for spending may be best but withdrawing cash on a Mexican credit card would probably not be a good idea even if it were an option?

For cash my go to places are https://tavex.pl/ and Diament. The rates for MXN to PLN don't look too bad actually, but if they want to exchange a huge amount they had better contact the bureaux in advance

However if they are able to get USD (or even euros) in Mexico at close to spot it may work out a few % cheaper. Interestingly Tavex seems to pay a higher rate for USD $1 notes