Is it a faux pas to offer a favor to a professor who has written you a letter of recommendation / served as a reference? I have a long and very good relationship with a previous professor of mine. I took two of his/her courses in college, got A+'s in both, and spent hours chatting with him/her on various academic topics even outside the scope of those classes. In addition, he/she has written me two letters of recommendation, both of which have served me very well in my career, and functioned as a reference for several jobs I've applied for (and for all of which I was given an offer).

This professor recently again agreed to function as a reference for a job application, and in our email exchanged asked me if I could recommend any resources for him/her to use when making a new website. I provided said resources, and also offered to make the site for him/her if they wished (I have several years of experience as a full stack web developer and would be able to make them a high quality site in a relatively short amount of time).

My question is this: did I put this professor in an awkward situation by offering to make the website for him/her? I understand that writing letters of recommendation and serving as a reference is an expected part of a professor's job, and I am worried that by offering my services free of charge, I may be putting the professor in a potentially questionable ethical position (i.e. making it look like writing me a letter of rec was a quid pro quo). Of course, the reality is that I'm just happy to help someone whom I greatly respect and consider a friend, but I'm worried that it may not be perceived as such.
This may vary greatly from person to person, but I, at least, would feel it was inappropriate to accept such a favor from somebody who I had written a recommendation for (unless it would already have been appropriate between us otherwise).

Personally, I feel that writing letters of recommendation for good people is not a favor, but a responsibility, a duty, and a privilege.  It is a responsibility and a duty because it is one of those little pieces of service that is vital to supporting the scientific endeavor and maintaining its quality.  It is a privilege because my opinions get to help shape the future of the field, and by supporting people who I believe in, I get to help make it the environment that I wish it to be.

In short: gratitude from a recommendee is appropriate.  Quid pro quo is not.  I would not hold it against somebody who offered a favor in return, but I would politely decline.

As I said, however, opinions on where the boundaries lie vary.  For example, I would not have requested information about web resources in the same email, as the professor did for you.  I personally think that is a faux pas (but, as noted, I'm pretty hard-line in my opinions here).  Thus, in responding, I don't think that you committed an error, though I would find the whole interchange a bit dubious myself.