Both sentences are equivalent in meaning but you should keep in mind that there is such a thing as putting in more commas than you need. (b) can be rewritten as:
One of my friends who works at your office, told me that.
One of my friends, who works at your office told me that.
(Although I believe this is less than ideal and would require adding in some more words after the comma)
Also consider revising both sentences into
A friend of mine who works at your office told me that.
Which sounds much more normal and idiomatic(something an English speaker is likely to say)
As for your second question:
Does either sentence imply that I have only one friend who works at your office?
This is debatable. The main verb in this sentence is 'told me', so the 'One'(subject) in this sentence may imply that you have many friends who work at your office, but only one amongst them told you 'that'.
In short, the sentence does not provide information about how many friends work at the office