Gaining

So basically we're wondering which is better
Heavy cream (330ml) mixed with some ice cream and a bit of nutella (whole gives 500ml, so kind of milkshake) or heavy cream alone for a night?

Make sure they can handle the cream. You can add a couple of gainer shakes/smoothies a day. The added nut butter is great, chocolate syrups are a good addition as is condensed milk.

She said that she can handle it... Anyway, we're on a low budget and she doesn't want to blown up or something, she wants to be in a normal weight... Which she said would be more or less 10lbs lower before being overweight

Just have a calorie surplus and start with a little amount of heavy cream to learn what their body can handle. Best to do small amounts throughout the day, but if they drink too much cream, they will feel bloated and not want to eat regular food and it will be counterproductive. Going for efficiency is a good way to efficiently get glued to a toilet.

And they can mix heavy cream with whatever, just as long as it’s enough for their body to handle and it is pleasant. Classic sodas and fruit juices pair well. The last thing they would want is to not enjoy the gaining part. Burnout will happen if things are unpleasant.