you dont actually say which vehicle was merging with the traffic flow,if it was the lorry then he would be at fault as most places where vehicles merge are at giveway details...even motorway slip roads where they joing the motorway are give ways(see the highway code) if however it was your boyfriend merging,then due to the size of the lorry and its amount of blindspots on the left hand side of the vehicle he probably didnt even know he had been hit.
A lot of car drivers tend to force their way out on these slip roads and are quite cocky that the vehicle will give way to them whereas with lorrys if they dont know your there then theyre not likely to move or stop for them and the blame cant be placed at the drivers actions,,,give way means give way to oncoming traffic,not take it for granted they willmove over or let you out