Errors and Unnatural Language in a Translation Challenge
Overall both translations are fine and fluent, with some small issues below:

Grammatical/Syntactic issues:

I think there is only one issue, in (2) 我们想酒保因为要省钱的. Either use 要省钱 as verb (i.e. remove 的), or use 要省钱的 as adjective (i.e. add 是 before 要).

Semantic issues:

In (1), margarita mix is translated into 玛格丽特混合物. In Chinese, people don't refer food/drink mix as 混合物. You can either just say 玛格丽特, or use the uncommon but well-understood word 混饮.

In (2), "ended up in the hospital" is translated into 住院. 住院 means hospitalization (overnight), while "ended up in the hospital" might be just seeing the doctor. It's fine if 住院 is deduced from contexts not shown in the question.

In both sentences, 酒保因为要省钱，所以... sounds a little weird, because 因为..所以 not only states causality, but also has a weak implication that the reason is valid, as if "if the bartender is poor, it is right for him to adulterate the drink." Using 为了 is much more idiomatic, e.g. 奸商为了赚钱，良心都不要了。

Also "they" in the last sentence is referring to the bar as a whole, not just the bartender, so 酒吧 would be more accurate than 酒保.

Idiomatic/Naturalness issues:

In both translations, 我们认为酒保.../我们想酒保... 认为 sounds too formal; 想 sounds unnatural since 我们想 usually means "we want to...". 我们觉得 or 我们猜 are better options.

My version:

你最好别去那个酒吧。我朋友以前去过一次，喝了一杯酒就去医院了。我们觉得酒吧为了赚钱，在酒里掺了化肥。

Notes on word choices:


最好 is optional. With it the tone is softer and more suggestive.
别 vs 不要: people tend to use 别 in Northern China and 不要 in Southern China. Both are correct and natural.
省钱 vs 节约成本 vs 降低成本 vs 赚钱: 省钱 is fine but a bit strange (what money is being saved and why, the ultimate goal is actually 赚钱 not 省钱); 节约成本 is a commendatory word so inappropriate (not a problem if you say it automatically in colloquial language); 降低成本 is clear but too neutral to condemn the misbehavior; 赚钱 is the best fit and indeed the most common word to describe a profiteer's motivation.
酒 vs 玛格丽特/玛格丽特酒: 玛格丽特 seems redundant because it is irrelevant to the point. The line sounds more fluent without it. 
把A放进B vs 在B里掺了A: former seems to imply intentional poisoning; latter is a common phrase to describe adulterated product especially food/drink.
肥料 vs 化肥: they are essentially the same, but when put side by side, 肥料 focuses on nutrition value while 化肥 focuses on chemical ingredients, hence 化肥 sounds more inedible and toxic.