They are the same material
A lab-grown diamond is a diamond. Same carbon structure, same hardness, same brilliance. A gemologist cannot tell them apart by eye — it takes specialist equipment reading growth patterns. Anyone who tells you lab-grown is "fake" is either misinformed or selling you something.
The difference is origin. One formed underground over roughly a billion years. One formed in a chamber over a few weeks.
Where they actually differ
Price
This is the big one. A lab-grown diamond typically costs a fraction of a natural stone of the same size and quality. Same budget, substantially larger stone — or the same stone for considerably less.
Resale value
Natural diamonds hold value better. Lab-grown prices have fallen steadily as production has scaled and may keep falling. If you think of a ring partly as an asset, that matters. If you think of it as something she wears every day for forty years, it matters far less than people suggest.
Rarity
A natural diamond is finite and genuinely ancient. Some people care about that a great deal, and that is a completely legitimate reason to choose one. It is not irrational to want the older thing.
Environmental impact
More complicated than either side claims. Lab-grown avoids mining but is energy-intensive, and how clean it is depends entirely on the energy source. Modern mining is far better regulated than its reputation suggests. Be sceptical of anyone offering you a clean answer in either direction.
How I would actually advise you
Choose lab-grown if size matters most to you, you want the maximum stone your budget will carry, or you care about the piece more than the provenance.
Choose natural if resale or long-term value matters, the origin story carries emotional weight for you, or this is a piece you expect to pass down.
Either way — spend on cut before size. A well-cut smaller stone outperforms a poorly-cut larger one in every light in the room. That is true of both types, and it is the advice most people ignore.
What I work with
Both, plus colored gemstones. When we talk I will tell you honestly which I would choose in your position, including the times when that means spending less than you planned.