Firstly, if the school was teaching phonics properly they would not give the child books to read which contain phonic 'knowledge' which hasn't yet been taught! That's what decodable reading schemes are for; pracice and consolidation of what has been learned so far.
In terms of using context, I wasn't really thinking of very young children just beginning their reading journey, but more for intermediate readers, reading books that don't have such carefully controlled vocabulary.
Of course, a child could well encounter this situation in their out of school reading. In which case the person reading with them needs to say something like "In this word the 'ear' spells and /air/ sound. You haven't learned about this yet at school, but you will." Then ask the child to sound out and blend the word with the /air/ instead of /eer/. This sort of incidental teaching might be enough for the child to remember in future that 'ear' can spell 2 sounds and to try both when the word isn't immediately apparent. Even more so if they have already been introduced to 'alternative' sounds for discrete graphemes. Even if they don't remember this has helped to advance their understanding of how the English alphabetic code works and the identification of new words is still firmly related to sounding out and blending.
I think you have replaced using context here, with person reading with them. In which case the person is helping them reach the correct word and they do not need to use context. Whilst I am all for people listening to children read and using opportunities for incidental teaching, there will come a time when they will be on their own.
(Perhaps my use of the bear example led you to think I was talking about very young readers - it was just in my mind because of a thread on AIBU, where a supply teachers could not convince a Y2 group that bear said bear; they were adamant it said beer.)
If the child is directed to guess the word from a picture or context they have not advanced their learning one tiny bit. Or, they have 'learned' that phonic knowledge is unreliable and that guessing does just as well. They may even start to be discouraged at this point because the letters didn't do the job they have been taught that they do and nobody seems to be able to explain why.
G