Taking the piss - WWYD

You could raise the issue of the law and child minding with your DD. Anyone looking after children could be breaking the law
Family members (i.e. grandparents, aunts, sisters, half-brothers etc) do not have to be registered. Any non-blood friends who look after your children must be registered if they look after your child for longer than two hours a day, at any time up until 6pm. Though if they care for them for less than 14 days per year, they may not be required to be registered or inspected, providing they inform Ofsted in writing at least 2 weeks prior to caring for your child.
Mumsnet have a discussion about this.
It could give your DD a way out of this if she is prepared to take it.

One of the reasons why she puts up with it is that one of the boys is a close friend of her lad and they have great time together. But it is taking its toll on her.

Also my DD and her OH are quite well off (but they work their tripe out to achieve this) and people do seem to take advantage of them.

I guess my role it to bite the tongue!

Very difficult though, especially if the DD isn't keen to be that assertive. Say something yourself and you risk the friend taking offence, upsetting DD and causing friction between the two of you.

Good luck!

Is the friendship worth all the work?

If she is a true friend, she will see that she is taking advantage and may reciprocate by offering to have your DGC - if she is not a true friend then she may 'flounce' - but would that be a great loss?

She has to say no, in the strongest terms she can. I'm sure you can support her in doing this.
My DIL was asked to mind a friend's daughter for an hour one morning, while the friend had a Drs appointment. Later that afternoon the friend returned, having taken the opportunity to go shopping! It didn't happened twice!

She is indeed! And I have to stand by and watch! (sits on hands/bites tongue) - thank you for the very helpful link.

WWID?
I'm not sure. Your DD will have to say 'No' politely but firmly because this could happen at every school holiday if she's not careful.
If this person offers to have your DGC during half-term so that she can get on with her own work and there is a fair reciprocation then that would be an arrangement that could be suitable.

I don't know if you should say anything - but you should certainly encourage your DD to be more assertive and to begin to say No in a positive but nice way.

www.assertiveness.org.uk/not_assertiveness.html#.WGvsb9SLRkg

There may be some tips in the link above.

But - lunch and tea - and for her too? She is really taking advantage of your DD's kind nature!

Sorry - grammar a bit garbled - I am feeling quite concerned about this.