What's that, Lassy? Timmy fell into a well?
Joking aside, the easiest for your ally spirit would be to use a complex action to switch form, provided it has a form actually capable of speech.
The base power Sapience does not grant the ability to speak, nor does Sense Link, which is, in any case, limited to the summoner.
I would further argue that Realistic Form gives the realistic form of a dog, which cannot speak either. This rules out the "out of the box" powers.
If you haven't invested the points in either a form capable of speech, an additional power like Mind Link (provided it exists in the list of spirits your character can summon), or a spell like Mindlink (provided your character knows it), it will not be able to speak.
All that being said, that's probably an extremely intelligent, and more importantly sapient dog there. It could easily scratch written instructions or a picture with its claws.
PS: It's "catch", not "caught"