The #1 piece of advice is to "Do the Math". Make sure that for their intended dining itinerary, it makes sense.
Be sure to add any subjective value of the plan being pre-paid and (somewhat) restrictive in there as well. For example, since I'm ok with the restrictions (mainly on locations, which the vast majority do take the plan, as well as the entitlements per meal) and not very good at saving the lump sum of money that would be needed, I put a good $50 to $100 value on the pre-paid nature of the plan. So, if I do the math, and come in better than $50-$100 short on the plan, I call it good for me.
Others do the opposite, and feel the restrictions are too much for them (or they don't really want to eat that way). They'll put say a 5-10% negative on the plan, so that after doing the math, if it doesn't save them at least 5-10%, it's not worthwhile for them.
It's all about identifying if a plan is right for your trip, and then which one fits the best. Don't try to fit your holiday to the plans, but rather see if one fits your trip. Included in my "Do the Math" is also to determine that the plan is how you intend to dine in the first place. If you (or them) are going to go "off plan" for much of the trip, it's usually better to just pay OOP for the meals. You can put the money onto a Disney/Visa Gift Card, charge the meals to the room, and pay the room with that Gift Card if you prefer to "prepay" for the meals, but the plans don't fit.
If they decide it's a good deal and just looking at
DDP (normal, 1TS/1CS/1Snack), aim for the pricier meals and try to keep the majority of the TS for dinner with CS for lunch. Avoid Signatures and other 2TS experiences for the most part (depending on length of stay, 1 or 2 may not hurt, but that's only if the trips are longer).