Grintovec in winter is something completely different from summer. First you need to know it's quite demanding fitness-wise - lower cable car station at 800m a.s.l., meaning 1800m ascent on the shortest route. You've been up in summer, so you know. Now add cold, snow and track not knee-friendly, especially if icy. Adding crampons spices it up more, but can't go without. Winter path to Kokrško sedlo hides a major danger to take seriously - high slip and avalanche risk just below saddle. Best in winter to stick fully to the right side, less exposed path to saddle. Track there too, visible if not drifted.
Then second part, somewhat more pleasant for walking (at least for me), but three spots needing much attention and ice axe knowledge. First: crossing under walls few hundred meters after junction sign to Skuta or Grintovec. Slip here = fall into basin below, no rescue. Second similar danger at crossing to Ovnov čr and Grintovec roof. Flatter there, but leads straight to abyss below. Ice sure during your planned trip, advise much caution. Roof path basically not demanding (except fatigue, which is usually very present here). But precisely here accidents have happened in the past due to slips - most choose crampons-poles combo for easier walking, which is true, but in case of slip with pole you can't do much. Better ice axe, in this case legs take most of the work and effort is somewhat greater. Depends on conditions and your own knowledge what you will do. On descent slip risk still present, additional trouble is possible loss of orientation in poor visibility, common on roof. Need to stick to left side, but not too much, because winter cornices on the very edge can collapse quickly.
Can't advise you either to go or to stay down. My personal opinion is that Grintovec perhaps isn't the best winter choice if neither of you has much experience and hasn't been up here yet. On the other hand it's definitely one of the nicest winter tours in KSA, worth doing under certain conditions. Decision is yours, decide by your own judgment.
If some more experienced colleagues perhaps add something (Janez, Juš, ...) or correct me somewhere, all the better.
Safe steps, wherever you go.
Jure