This is a typical example for a drilled window frame (in this case made for mounting a curtain rail):
It is a very common problem in rented apartments that tenants, normally without ulterior motives, drill through window frames in
order to install some curtains or the like.
The "rude awakening" follows often as the tenant moves out: Landlords see their property damaged and demand compensation or try to make some exta money by retaining the deposit.
2. The simple Solution: Seal the drilled hole with Silicone
A very simple and cost efficient solutin to repair and/or seal the drill hole on a window frame is with Silicone.
An advantage of this solution is that the sealing is barely visible and is compatible with all kind of drill holes.
Additionally it seals the often put forward argument of a "thermal bridge".
All you need is:
1x White Silicone Sealant/Glue (e.g. quite affordable:
DAP white Silicone on Amazon.com)
Important: DO NOT use transparent Silicone (visible); also all kind of Acryl adhesives are NOT recommended because of high capacity loss over time.
First you have to cut open the silicone cartouche with a knife and insert the silicone cartouche in the silicone gun.
Then simply position the pistol tip slightly above the drill hole and fill in the silicone material in the drill hole.
Subsequently just wipe off the overlapping silicone material so that a even surface is formed and let it harden for about 1 day.
Finally please take care that all other parts of the window frame are free of silicone material after you have finished the sealing procedure.