Pro rata rights don't apply to notes. Are they are asking for a super pro-rata right -- that is the right to do more of the next round? You cannot do that in a scalable and consistent way and it sets up a bad precedent for other note investors and the institutional investor that might want to lead a round.
Let me make sure we're clear on what a pro rata right is. A pro rata right says that for whatever percentage of a company an investor owns, they get to do that percentage of future financing rounds. So if an investor owns say 10% of a company, and you raise $5m in a Series B, they get to do $500K of that $5m.
Now, when you are issue notes, the notes are debt not equity. There are no "pro rata" rights on debt since you don't own any percentage of the company yet. You will own equity once the note converts and it will convert on the terms of the equity lead investor, who will specify pro rata rights.
At this stage, the investor may be asking for "super pro rata rights" or guaranteed rights to do a percentage of the next round. This screws up fairness to other investors and can make it very difficult for an institutional investor to lead the next round since they will have their requisite ownership needs (e.g. 20 - 35%) and a super pro rata right could screw with that.