How you deal with this should consider your wife’s state of mind. You have mentioned that she is ill.
If your wife has a low self esteem because of her illness, feels unattractive because of it, the LAST thing you should do is say something to your wife, because no matter how you phrase it she will hear: “honey, I have the hots for your best friend”. It will devistate your wife. She will take it as all her fault because she is sick.
The best idea is for you to just be away from this woman. NEVER permit yourself to be alone with her.
OTOH, if your wife is self confident, knows that you love her and find her attractive in sickness and in health, then she would be able to take the “honey, your friend has been giving me the eye and I do not think I should be around her, it is causing me to have lustful thoughts”.
Overall, have you looked into
www.e5men.com? Fasting for your wife and your marriage is a powerful thing.