My answer assumes that Bob can not predict HOW things will change, just that they will change, in subtle ways. Also, I am assuming that Bob can and will voluntarily answer your questions. I am also assuming that this works for any PREDICTIONS Bob makes. That is, any prediction that he makes will never be true. I am also assuming that Bob, in god faith, can tell the truth as it was at the time of him saying it.
I am not sure if the issue is in not being able to trust that the answer Bob gives you is the truth, but in being able to absolutely trust that the answer Bob gives you is NOT the truth.
If a betting man knew that Bob's answer could always be trusted to ultimately NOT be true, then a betting man could make a lot of money.
'Bob, what team will win the Series?' Then bet AGAINST the team he says. 'I will bet you that your favorite team x will NOT win the Series'. 'Oh, come ON, they are sure to win the series. You're ON, man, you're ON'.
Of course, a person with criminal intent would WANT him to tell the truth, so that it would become the UNtruth.
'Bob, try that door and tell me if it is locked?' In which case, you WANT him to tell the truth, that it is locked, so that it then becomes the UNtruth.
'Bob, is that watchman who is looking at us paying attention to us?'
'Bob, is that merchant charging for his goods?'
'Bob, will you tell the judge the truth about my guilt?' It doesn't matter if the judge believes him or not, but as soon as Bob says I am guilty, I am no longer guilty.
It would seem to me, that the utility and the advantage of having Bob as a friend would not be in what Bob says is the truth, but what HAPPENS in response to his telling the truth. That it reliably and immediately becomes the UNtruth in some way.
In day to day conversations as a friend, one could easily accommodate his eccentricities, if one knew absolutely that whatever he said, even if said in good faith, would be wrong.
'Bob, how much money do you have?' Bob: 'I have ten dollars'. So, if Bob answered truthfully in good faith, you know he has approximately ten dollars, but not exactly ten dollars.
'Bob, what time is it?' Bob: 'It is ten o'clock'. So again, if Bob is answering truthfully in good faith, and tells you the correct time, then you know it is AROUND ten o'clock but not exactly ten o'clock.
If he always answers with an approximate answer, that is close to the truth, you of course can always accommodate. The trick is to learn to ask Bob the question in the right way.
So those who perhaps find utility in Bob, and want to capitalize on his powers, but also want to have a good relationship with him on good faith, would have no problems as long as they always knew his good-faith answers were APPROXIMATE answers, and his devious faith answers were always the truth as he knew it at the time of his response. You and Bob would know the answer would be immediately incorrect as soon as he gave the response.
And, of course, there is the situations along the lines of: Bob: 'I need to go to the bathroom right now' means that, if it were the truth, Bob NO LONGER has to go to the bathroom right now, but he WILL have to go to the bathroom AGAIN (is that the right term) shortly.
in such a way, as long as the other person wanted to have a relationship with Bob, and Bob always responded in good faith with the truth, but both of you knew it was no longer the truth but approximately the truth, the relationship would work out.
So, really, it is about how much the OTHER person wants to have a good relationship with Bob, and is willing to accommodate, provided Bob enters the relationship in good faith.