OK.. this is simple. Your boyfriend isn't in anyway responsible for the damage to the iPad, either legally or morally, since the father was the one who negligently entrusted the iPad to a toddler.
Of course that doesn't help when it comes to your boyfriend's relationship with his father, but I guess that's a whole different set of issues.
Probably the best thing to do is for your boyfriend to calmly tell his father that he's not responsible and that's the end of the matter. If the father gets mad, your bf should stay calm, and repeat that it's not his responsibility. As father raises his voice, bf should lower his, forcing the father to stop shouting to listen. Don't rise to the bait. It will be emotional for your boyfriend - nobody likes fighting with their dad, so you job is to calm, comfort and support him.
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AQ:37 FQ:105 ENTP
Your Aspie score: 141 of 200
Your neurotypical (non-autistic) score: 78 of 200
You are very likely an Aspie