Based on your description, this would appear to be a foul by the defender. 17.8 talks about the opponent being “unable to avoid” – it would appear that the defender could have avoided the offense player, but they just weren’t looking – that is not a valid excuse for not avoiding someone.
Please refer to section 12.7, 12.8 and the interpretation for section 17.8.1 for more information regarding this:
http://rules.wfdf.org/rules/ultimate/17-12-receivers-and-positioning
http://rules.wfdf.org/rules/ultimate/fouls