To what extent is curcumin destroyed by boiling?
Googling, I found this report, presenting a chemical and technical analysis of curcumin. Skimming through it, it seems that degradation of the substance should not be a problem due to temperature, although the report doesn't say much about it except that it shouldn't be a problem:

Curcumin is stable in dry food. It is relatively stable to heat so it can be used in thermally treated foods.

However, it warns about using it in solutions of a high pH, without stating which of the degradation products may be unfortunate to ingest. Furthermore curcumin is sensitive to light, so if you want as little as possible of it to degrade it is probably a good idea to keep a lid on the pan while you're heating the solution and consume it fairly quickly afterwards:

In native form curcumin is not suitable as a colouring agent in aqueous solutions of pH &gt; 7
The principal colouring components of curcumin are not particularly stable to light, especially in solutions. After the photo-irradiation of compound 1, a cyclisation product was detected, as well as decomposition products, such as vanillic acid, vanillin, and ferulic acid (Sasaki et al, 1998). Commercial formulations of curcumin are available that are designed to minimize the inherent light instability.

With regards to the pH of your solution I don't think that should be a problem, as it is probably fairly acidic from juices of the lemon.