Just remeber the rate at which Tesla are developing new models as well as scaling up production and making iterations on-the-fly is very high indeed and IMO this process can probably not be sped up further by "throwing money at it" so to speak. A year-to-year growth rate in production of more than 50% per year for several years while introducing new models as well as keeping up very high qulity is quite rare when it comes to something as complex as a car. I think being involved intimately with a company like Apple at the moment would only risk slowing things down and making Tesla less agile. I would like to see them invest in the Gigafactory though, without interfering with the project, only being a financial partner. Like v12to12v said above: Apple has little to offer right now other than cash.
Cash, and an endorsement from a respected brand.