Great question! Your friend is correct, grammatically speaking. You don't need "tenga". Think about what you're trying to convey in English: when you say, "If I have a million dollars, I will buy a big house." Because you are using the future tense (will buy) in the second clause and present tense in the first ("if I have"), you are basically saying that the possibility still exists for you to one day have a million dollars. In other words, there is no doubt here, and therefore, no need for the subjunctive in English or Spanish.
I think the two tenses you really want to compare are 1) the pluperfect subjunctive and 2) the imperfect subjunctive:
1.) Si hubiera tenido un millón de dólares, habría comprado una casa grande.
If I had had a million dollars, I would have bought a big house.
2.) Si tuviera un millón de dólares, compraría una casa grande.
If I had a million dollars, I would buy a big house.
You may be misunderstanding the use of the subjunctive in English for this type of "if" sentence ("If _____ [condition], would _____________ [some form of conditional]). Remember that there are different uses for the subjunctive. This particular sentence structure you have provided is all about a hypothetical situation expressed by the past subjunctive or perfect subjunctive (because it's not a current reality and there is implied doubt of its likelihood of becoming a reality), followed by a hypothetical response in the conditional or perfect conditional.
Again, the meaning of this sentence (hypothetically having a million dollars) cannot be expressed by present subjunctive because this is an "If" usage of the subjunctive, meaning it always has to be put in some form of the past subjunctive (imperfect or perfect).
The only times that you would use present subjunctive ("tenga") would be for one of the following structures:
- Desire/volition - Mi mamá quiere que yo* tenga un millón de dólares
- Recommendations - Mi mamá recomienda que yo tenga un millón de dólares
- Value Judgments - Es necesario que yo tenga un millón de dólares
- Doubts - No es probable que yo tenga un millón de dólares
- Hope - Ojalá tenga un millón de dólares / Mi mamá espere que yo tenga un millón de dólares
- Emotions - Mi mamá se alegra de que yo tenga un millón de dólares
*Note that I have included the word "yo" in each of these examples, although normally this is not necessary. I have only included it here to highlight the change in subject (Mi mamá --> Yo) during the same sentence.
I hope this helps! Please feel free to reach out if you'd like to have a live session with me online sometime.
Alissa G., M.A. Spanish