Short answer:
Yellow broke rule 18.2(b).

Long answer:
Although there are times that rule 18 does not apply, such as when approaching the starting line to start and between boats on opposite tacks on a beat to windward (see the preamble to Section C and rule 18.1 for a complete list), a downwind finish is NOT one of them. Rule 18 DOES apply at gybe, leeward, and downwind finish marks.

The RC Boat is a mark (see the definition of mark). The two boats were overlapped (see the definition of Clear Astern and Clear Ahead; Overlap) when the first one reached the zone. The outside boat (yellow, on starboard tack) was required to give the inside boat (blue, on port tack) mark-room per rule 18.2(b). Mark-room (see the definition of mark-room) consists of "room to sail to the mark when her proper course is to sail close to it and room to round the mark as necessary to sail the course". Yellow did not give Blue that room, and therefore broke rule 18.2(b).

I hope that helps,
Eric