According to the PHB, page 250,
Until the spell ends, the target's speed is doubled, it gains a +2 bonus to AC, it has advantage on Dexterity saving throws, and it gains an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.
Since a Beholder's central eye is in front of it's body, and always pointing outwards (if on), it seems sane that a Beholder could have the spell cast on it (say by a minion). If hostiles outside the cone can attack it with spells, allies should be able to buff it.
Under the spell, the Beholder should get the bonuses to defense, but how should its attacks be affected? Many creatures have an action termed "multiattack," while the Beholder does not. Compare the Behir (MM p25) and Beholder (MM p28):
Multiattack. The behir makes two attacks: one with its bite and one to constrict.
Eye Rays. The beholder shoots three of the following magical eye rays at random (reroll duplicates), choosing one to three targets it can see within 120 feet of it:
Obviously the Behir should get only an additional bite or constrict while hasted (as mentioned here). Does the Beholder get 4 eye rays or 6?