This is, I'm afraid, a rather vague answer since it's a long time since I studied this and I no longer have access to the relevant journals. Anyhow, while the transition is forbidden for an isolated Europium atom it is allowed in the crystal because the light interacts with the crystal field surrounding the Europium atom so it's not a pure dipole interaction.
If your college has access to the Journal of Physics look up the paper by J. E. Lowther J Phys (C) 1974 7 p4393. Vague recollections and some Googling suggest this explains how to calculate transition probabilities including contributions from the crystal field. It's available online here.