This can certainly be a tricky situation to handle. Friendships of any kind have their own rhythms and life cycles. We make friends (if we are lucky) and those friendships develop and grow or perhaps never quite take off. Some friendships last a few months or years and then fade into the background; some end due to conflicts and misunderstandings; in other cases, people simply move away or grow apart. A few of our friendships may last a lifetime, even across vast geographic distances.
Lots of people have acquaintances at work who become “work friends.” Often they are co-workers and sometimes they are bosses or supervisees. Work friends may just talk at break times or have lunch now and then or they may become close enough that the friendship goes beyond the workplace. Workplace friendships that begin to involve time outside of work may remain casual or may become quite deep.
When there happens to be a similar level of interest on the part of both people, it works out very well. If someone becomes less interested, or moves into new friendships and activities, or simply becomes disenchanted with their friend from work, things can obviously become awkward. Sometimes the hints and repeated excuses about being busy can function as an effective message to the other person that he will pick up on and then choose to back off. He may or may not ask you what’s up. In most cases, it is probably a good idea to answer his question as honestly and tactfully as you can. There are also times when that may not be a good idea. If the person seems volatile and is likely to overreact, it may be best to say as little as possible.
One approach you might take with your friend is to let her know that you have recently realized that you prefer to keep your personal life and work life separate, that you value her as a colleague and work friend, but that you prefer to keep the friendship within the office. Of course, this may not be appropriate if you have other work friends with whom you plan to continue out-of-office friendships.
If you have tried to convey the message multiple times and he or she seems to not “get it”, you have a couple of choices: you can keep manufacturing excuses or you can set a time (probably outside of the office) to have a private discussion about the changes in your desires. There are more tactful and less tactful ways to convey this, but if hints aren’t working, a sit-down conversation is probably the way to go. I recommend tact and honesty as the best way for you to disentangle from the personal friendship and still keep the friendly working relationship intact.