Our current rent is $2600 I want to offer to pay $3000 for as long as she wants 1-15 years and then pay $600k My main question is whether that type of structure of offer makes sense legally or technically or practically. 

Here are some situations where it might make sense, along with considerations for how long one might wait for the situation to improve and what margins might be acceptable for a flip:Expectation of Significant AppreciationStrategic Value or Unique OpportunitiesMarket Rents Expected to IncreaseFix-and-Flip with No Rental StrategyConsiderations Before Buying with Negative Cash Flow:Financial Resilience: Make sure you have enough cash on hand to meet the shortfall without endangering your overall financial situation.exit Strategies: Establish practical, well-defined exit strategies.

They are not strict with tenants, they buy properties in tenant-friendly states/counties, and son on.I would bet you that if one of your RE 'friends' would come on here and show us their business practices, we would see a poorly run business.Anyway, never listen to the naysayers, it sounds like you are set up well and you're on this forum, which gives you a huge advantage.

. $1 vs $0 is practically the same thing for real life purposes. 

This practical experience can offer insightful information, networking possibilities, and mentoring.Attend Workshops or Seminars on Real Estate: Seek out workshops, seminars, or educational events on real estate that are being held by nearby organizations or investing clubs.

If you want to protect your friend's capital as if it were your own, you'd want to:1) Secure the loan with real estate in the form of a deed of trust or mortgage (depending on the state the property is in)2) Obtain property insurance binder with your lender as an additional insured3) Obtain a lender's title insurance policy protecting them (different than an owner's policy) and transfer all funds through an escrow closer or real estate attorney (depending on the state, the requirements for closing may differ)4) Use a real estate attorney who also practices private lender law in the state the property is located in.

While their online marketing would not have been legal when I started practicing, it is perfectly legal now. 

Hello! I am a real estate agent new to real estate investing and need some advice -
1. Which NAICS code should I use for my activities?
- Residential purchases, remodel/repairs, and re-sale (flips). I'll typically...

We might see buyer agent commission rolled into loans as a standard practice in the coming months and years.

There is no legal limit, but there is almost always a practical limit because a sponsor can only deploy so much money before they have to lower their underwriting standards etc.