Rules are more relaxed when using the Mar Menor, as it appears to be classed as an inland lake, nearest English translation.
Why worry about launching charges. If it was a big harbour like Aguilas or Carboneras, Almeria, the Harbour Control would be onto it immediately. They watch and patrol every craft.
International qualifications are not recognized in Spain, surprise surprise.
I quoted the example of my next door neighbour been Captain on cruise liners for thirty odd years. His over qualified credentials are not recognized by Spanish Maritime.
And, yes you are correct in your assumption of segregating English users.
I have Spanish friends at Garrucha Harbour and Villaricos Harbours and the prejudice is common knowledge.
You cannot launch a boat from these type of harbours without paying launching fees.
They even come round these harbours noting car registration numbers and chassis numbers of trailers. It is not uncommon to see the Guardia called in to offenders.
Sometimes however it pays just to get on with it.
Your friends qualifications, adequately qualified internationally, will not be recognized or accepted here in Spain. So as he has not been challenged in two weeks, carry on.
Probably okay on the "lake".