For the most part, the same kind of advice you give to any cyclist will suffice here. Encourage them to take it easy at first, to give them the opportunity to experience cycling--and specifically climbing--in a supportive, non-threatening environment.
Pick a good route
What hills and roads you pick will have an effect on how the newbie cyclist perceives the ride. Lots of traffic, or many wall-like steep hills, or narrow shoulders--these will all discourage the new riders. Find a route that builds up to steeper hills gradually, with wide, well-maintained shoulders. Perhaps even take it off-road at first. You want to avoid roads like this:
...rather, look for roads with wide shoulders or even bike lanes (they tend to calm cyclists, perhaps with a false sense of safety, but they'll let your friends concentrate more on the cycling and less on the traffic.)
(This was at the top of a long hill, if that's not apparent.)
(This hill was tough, but the lanes were wide enough for passing.)
How to use your gears
There's a strange perception that you have to "tough it out" to get to the top of a hill, that spinning up a hill in a very low gear is somehow demeaning. Make it clear that it's not only okay to go to the lowest gear available, it's what many cyclists do. Explain that a little bit of planning ahead when they see the hill approaching will put them in the right gear before they need it. Building up speed on the previous downhill (if there is one) will also help.
Wait it out
Another thing that will help is patience. If they have to walk a few hills, be supportive. Wait for them at the top, or perhaps at the next turnoff on the route. Over time, this will change. Consider walking up the hills with them. (Walking a bike up a steep hill is good exercise, particularly a heavier bike!)
Whatever your friends achieved that day, let them know it! This may go without saying, but tell them what they did well, and how they could have more fun next time. If, in the end, they decide hills aren't for them, then avoid them on your next ride.
All hills look steeper when viewed from a distance. You might suggest that they keep telling themselves that!