what happens to stock prices of EU companies if the Euro weakens?

Well, for companies that do a lot of exports such as Lamborghini, Ferrari, etc. their share prices are actually going to rise because most of their cars are bought in US Dollar from clients residing outside of Europe. If these companies get paid for their products in USD, they can convert this dollar back to euro and declare higher profit, pay dividends etc. which boosts their stock price.

However, companies that rely on local consumption of their products within the EU may see a drop in profitability if the Euro weakens because EU citizens will see most of their purchasing power eroded & may choose to go for cheaper alternatives to save cost.

If tthe weakening of the euro is caused by inflation, the ECB may have to hike rates thus making it hard for EU companies to access loans for expansion so they may declare less profits and the stock price will fall