Dozens of threads about this.
White-out aka Tippex was a simple solution back in the days of typewriters. Stationary/office supply places have sheets of self-adhesive marker dots in various sizes and colours. (from plain white to glorious pink glitter). Bits of any tape like your friend uses would be just fine too, just a bit messier, cutting out such a small piece if you want it any way regular - for one show it doesn't matter.
Permanent dots are not difficult either especially if you just want the 7th.
The only trick bit with permanent dots is if you want for example 5,7,9 is to get them lined up nicely - takes a bit of measuring and care. Otherwise it is just a matter of drilling a small hole and filling it with something light colored. Thin white plastic rod is the typical product used: put a bit of glue in the hole stick in the rod and cut it flush with a sharp blade. Sand it flush. Alternatively, mother of pearl dots can be used but then you have to make sure the hole is not too deep. Match sticks, thin cocktail sticks etc can also be used. Practice drilling on a piece of waste wood until your get the hand of it. If you have a local repair guy, it won't cost a fortune unless you want some sort of exotic marker such as a precious stone.