kirkjufellsfoss
I suspect it is the mountain Kirkjufell rather than the waterfall you mean, as the waterfall is insignificant. The mountain must have been used in some prominent publicity, or some new guidebook featured it prominently, because just recently lots of people say they want to go and see it, and in the past no one had ever heard of it. Either way, it is a good 2 hour (each way) detour off the ring road. There's much more of interest in Snaefellsnes, the region where it is located, if you do choose to spend some time in Snaefellsnes. But I do question whether you'd really want to extend your tour out there, especially given the limited daylight hours of October will tend to reduce what you can achieve the rest of the time.
they discourage the use of motorhome in late october cause is to cold
I suppose they don't want disappointed customers. The major issue is that the weather isn't very nice. And you will be stuck in the van as you are into the half of the year with more night than day, hope your friends are good friends. In October the average high in Reykjavik is 8C, av low 3C, and Reykjavik is a relatively mild location. Maybe you aer used to a place with a seriously cold winter and think that's nothing, but in the wet and the wind single digit temperatures can be very uncomfortable. Maybe ask if the camper van has heating and the practicality of using it.
You can suffer a bit of traffic disruption with some snow and storms, though much in the way of heavy snow is relatively unlikely in October, though probably you'll see a bit in a 10 day trip. Make sure you have enough flexibility to lose a day and still get back to Reykjavik, and keep an eye on the roads website vegagerdin.is weather forecast vedur.is so you know what might be coming up.