So yes, having read the section myself and seeing Shadowstep's quotes, a favor is not required unless they make a negotiation test. However, note that it says they will 'Normally' accept. They are not at all required to do whatever you say. Plus, you have to keep in mind that talking to contacts is primarily roleplaying. If your players are constantly bumming for favors without ever doing anything for the contact in return, you are completely in your right as GM to reduce that contact's loyalty as he starts to get annoyed by this behavior.
Also, you should note that the level of the favor is denoted by RISK. A Rating 6 favor is one that is a major risk to the contact. If it seems like your player intends to abuse his 6/6 friend, it's totally possible that performing this favor has caused some tragedy to befall him, reducing his Connection, Loyalty, or both. Hell, you could outright have him get killed if you want.
And again, there is the whole 'higher the connection, harder it is to contact him' problem, where 5 out of 6 times, the Contact isn't available.
All in all, this seems like a really hard thing for a player to abuse. as the GM, you have so many ways to nip this in the bud that it would do more harm than good to try.