What happens when your movement is set to zero while attempting to move through a friendly space?

Question

Say Anne is 10 ft away from and fighting Grogo the dwarf fighter, Grogo has Sentinel and Polearm Master. Grogo still has his reaction. Anne has a reach weapon, so she and Grogo are pretty much on a contest of who can skewer the other first.

Now Bets the Hired Sword wants to get in the fray and take a shot on that tough Dwarven skull. Bets is allied with Anne, so Bets can move through her space. Bets moves, and enters Anne's space, and takes a pike to the knee from Grogo. Now Bets has zero move, but haven't finished moving yet, and now has 0 move while inside an ally's space.

Assume all eight spaces around Anne are free. Bets moved into Anne's space from behind (the one opposite from Grogo's).

Where does Bets end up?

Answer 1

Bets ends her turn in Anne's space

The first step of the solution is straightforward out of the PHB, on page 191:

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

From earlier on that page, it's worth noting that Anne's space was difficult terrain for Bets, but that doesn't really matter now that she's stuck. Since she was forced to stop moving at this spot, and it was not of her own volition, the movement is valid and her turn ends in Anne's space.

Bets can continue to act

Although her movement is zero, her turn has not ended. If she can perform any actions from here, she is still welcome to. She can throw her weapon, drink a potion, or perform any other actions that are legal from her current space. The DM would be free to decide if certain activities would have disadvantage while she is so close to Anne, but that is a separate matter.

Anne must move on her turn, if able

Although Bets has no choice about where she ended up, once Anne's turn comes around Anne must move from there before the end of her turn if she is able, and assuming nothing has moved her or Bets in the meantime. The same rule from above means that even though Anne started her turn sharing a space, she cannot casually disregard it. She must move to another space, even if that would provoke an opportunity attack, trigger a known trap, or just be tactically disadvantageous.

Answer 2

Bets and Anne are in the same space.

PHB 191 states, (emphasis added)

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

However, Bets did not willingly end her move--she was forced to by the opportunity attack. Since the rule forbidding overlapping spaces only refers to willing movement, it's valid for Bets to end up in Anne's space.

Answer 3

Bets is behind Anne and is 15 feet (three squares) from Grogo

Why?

Turn it the other way around. If Bets was within Grogo's reach and had tried to leave, Grogo could have taken an attack of opportunity. If the attack hit, the Sentinel feature would have stopped Bets 10 feet from Grogo, not 15.

"The attack occurs right before the creature leaves your reach." [Player's Basic Rules v0.3 page 73].

So, when moving in the opportunity attack occurs right before the creature enters your reach.

In the situation asked above, before Bets moves into reach, Grogo's hit cancels Bets' remaining movement. Bets can still continue the turn, performing Actions and/or Bonus Actions if they are available. The only effect of Sentinel is that Bets can't move any more this turn.

What if Anne was 15ft (3 squares) from Grogo?

First, neither Anne nor Grogo would be able to attack each other.

When Bets moved, Grogo's Sentinel attack would stop Bets in Anne's square. This means Bets is forced to end the turn in another creature's space unwillingly. Until one of them moves away, Anne and Bets are now squeezing together in the same space [Player's Basic Rules v0.3 page 71]. Both have disadvantage on attack rolls and DEX saving throws; attackers have advantage to attack them both. In short, Bets' action has left Anne and Bets in a tactically poor situation.