I honestly wouldn’t worry about traffic if ‘your friend’ is on a motorbike – in all my times up in the NW of Scotland it hasn’t been an issue, folk are usually great about using the passing places if they see bikes coming. The weather’s the thing that makes or breaks a trip and I’ve had it all – particularly because we always like to book the accommodation in advance. Chances are you will get wet but will also see sunshine. May / June’s usually my preference.
I’ve also been up that way with my son in our MX5 for the August Bank Holiday weekend. Lots of midges, stunning weather and actually not too much traffic surprisingly. Scotland’s a big place with not too many folk in it away from the cities so I think you notice congestion more if you’re in something like a massive motorhome rather than on a bike or a small car.